Phosphorylation Site Necessary for Activation of the Raf-like Protein Kinase ARK, a Positive Regulator of Abscisic Acid Response

Abstract

Abscisic acid (ABA) is a plant hormone important for environmental stress responses. We identified the Raf-like protein kinase ARK as an upstream regulator of subclass III sucrose non-fermenting1-related kinase2, known as a key activator of ABA signaling in Physcomitrella patens. Phosphopeptide mapping revealed that ARK is phosphorylated at a specific Ser residue in the activation loop of the kinase domain. Substitution of the Ser residue to Ala caused a decrease in ABA-induced gene expression while that to Asp caused an enhancement of the gene expression. From these results, we conclude that phosphorylation of ARK is important for its kinase activity and a response to ABA.